Mechanism and method of multiple documents feeding detection

ABSTRACT

The present invention provides a multiple documents feeding detection mechanism includes an advancing roller applying a driving torque, a retarding roller applying a resisting torque and connected to the advancing roller for transporting pieces of documents, and a detecting means. The friction of a surface of the advancing roller, the friction of a surface of the retarding roller, the driving torque and the resisting torque are tuned to urge the retarding roller either into rotation while one signal document fed into a nip between the advancing roller and the retarding roller, or in standstill while multiple document fed into the nip. The detecting means determines whether the multiple documents feeding is caused by monitoring an action of the retarding roller.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multiple documents feeding detection mechanism, more particularly, to a mechanism and a method of multiple documents feeding detection capable of detecting multiple documents by means of adjusting the action of a retarding roller.

2. The Related Art

Nowadays, image processing products, such as printers, scanners, fax machines, copy machines, are more and more popular on the current market for the purpose of automatically and sequentially processing a stack of documents. A multiple documents feeding detection mechanism is mounted on the image processing products mentioned above to detect the multiple documents feeding, and make sure the stack of documents is sequentially processed.

A conventional multiple documents feeding detection mechanism is disclosed in U.S. Pat. No. 6,212,130 which discloses an ultrasound detection mechanism. The ultrasound detection mechanism comprises an ultrasound beam generator located on the upper side, and an ultrasound receiver located on the lower side of a transport path of the image processor products respectively.

The ultrasound beam generator generates a beam of ultrasound energy radiating through the transport path. The ultrasound energy beam is received by the ultrasound receiver. While the ultrasound energy radiates through the air gap along the transport path, it will cause a weak attenuation. While the ultrasound energy radiates through a signal document introduced into the transport path, it will cause a general attenuation. While the ultrasound energy radiates through the multiple documents introduced into the transport path, it will cause a strong attenuation.

Therefore, the ultrasound detection mechanism determines the kind of the multiple documents introduced into the transport path by monitoring the magnitude of attenuation of the ultrasound energy.

However, the conventional ultrasound detection mechanism has some defects. Firstly, it often makes an incorrect judgment when the ultrasound energy radiates through a thick document introduced into the transport path, such as an IC card and a credit card. So it often causes an abnormal strong attenuation of the ultrasound.

Secondly, the precision of the ultrasound receiver is likely to be interfered by the ultrasound beam generator or the dusts on the cover of the ultrasound receiver. Thirdly, it is inconvenient to mount the ultrasound beam generator on the upper side of the transport path and the ultrasound receiver on the lower side of the transport path. Fourthly, the cost and the design for the ultrasound beam generator and the ultrasound receiver become more expensive and complex.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a multiple documents feeding detection mechanism having an advancing roller with a first surface, a driving means, a retarding roller with a second surface, a clutch and a detecting means. The driving means provides a driving torque to the advancing roller. The clutch provides a resisting torque to the retarding roller.

The retarding roller connects to the advancing roller for transporting one piece of the document. The friction coefficient of the first surface is greater than the friction coefficient of the second surface, to make the friction coefficient of the second surface be greater than the friction coefficient of the document. Thus, the driving torque will be greater than the resisting torque.

The retarding roller is urged into rotation while there is a single document fed into a nip between the advancing roller and the retarding roller. In stead, the retarding roller is remained in standstill while multiple documents are fed into the nip. The detecting means detects whether multiple documents feeding occurred by detecting an action of the retarding roller.

A further object of the present invention is to provide a multiple documents feeding detection method comprising the following steps.

-   Step 1: providing an advancing roller applied a driving torque, and     providing a retarding roller applied a resisting torque, the     retarding roller is connected to the advancing roller for     transporting one piece of document; -   Step 2: setting the friction coefficient of a surface of the     advancing roller be greater than the friction coefficient of a     surface of the retarding roller, to make the friction coefficient of     the surface of the retarding roller be greater than the friction     coefficient of the document, and the driving torque will be larger     than the resisting torque for urging the retarding roller into     rotation when the multiple documents be fed into a nip between the     advancing roller and the retarding roller, the retarding roller will     be remained in standstill; -   Step 3: providing a disk with a first tuning portion and a second     tuning portion, driven into action with the retarding roller; -   Step 4: radiating a beam to the disk; and -   Step 5: determining multiple documents fed into the nip according to     a received constant intensity of the beam tuning by one of the first     tuning portion and the second tuning portion due to standstill of     the disk, and determining a single document been fed into the nip     according to a received variable intensity of the beam alternately     tuning by the first tuning portion and the second tuning portion due     to movement of the disk.

Therefore, the multiple documents feeding mechanism determines multiple documents feeding by detecting action of the retarding roller to be urged into rotation or remained standstill by comparing the friction coefficient of the advancing roller and the friction coefficient of the retarding roller, as well as the driving torque and the resisting torque.

Hence, the merit of the multiple documents feeding mechanism is that the thickness or amount of the document fed into the multiple documents feeding mechanism can be much more than ever. By means of monitoring the movement of the retarding roller to determine the multiple documents feeding can avoid the influence of dust, conveniently used to integrate with transportation means of an imaging product, and that the cost thereof is cheaper than the cost of the ultrasound detection mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be apparent to those skilled in the art by reading the following description of a preferred embodiment thereof, with reference to the attached drawings, in which:

FIG. 1 is a cross section view showing a scanner with a multiple documents feeding detection mechanism according to the present invention;

FIG. 2 shows a status of no document being transported into a nip between an advancing roller and a retarding roller of the multiple documents feeding detection mechanism according to the present invention;

FIG. 3 shows a status of a single document being transported into the nip according to the present invention;

FIG. 4 shows a status of multiple documents being transported into the nip according to the present invention;

FIG. 5 shows an exemplary of a detecting means of the multiple documents feeding detection mechanism according to the present invention;

FIG. 6 shows an input signal seen from a sensor of the detecting means and a corresponding output while the retarding roller is in rotation according to the present invention;

FIG. 7 shows an input signal seen from the sensor of the detecting means and a corresponding output while the retarding roller is in standstill according to the present invention;

FIG. 8 shows another input signal seen from the sensor of the detecting means and another corresponding output while the retarding roller is in standstill according to the present invention; and

FIG. 9 shows another exemplary of the detecting means of the multiple documents feeding detection mechanism according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The object of the present invention is to provide a multiple documents feeding detection mechanism 100 applied to various imaging processing products, such as printers, scanners, fax machines, copy machines for detecting the multiple documents feeding. Please refer to FIG. 1, it shows an exemplary embodiment of the multiple documents feeding detection mechanism 100 applied to a scanner 9.

The scanner 9 defines a transporting path 90. The scanner 9 includes a document tray 91 arranged at a front of the transporting path 90, a document discharging tray 92 arranged at a rear of the transporting path 90, a document picking up means 93, a document feeding means 94, a document transporting means 95, a scanning means 96 and a document discharging means 97. The above elements are sequentially arranged along the transporting path 90.

A stack of documents is placed on the document tray 91. The document picking up means 93 picks up one piece of document from the stack of documents. The document feeding means 94 feeds the picked up document into the transporting path 90. The document transporting means 95 transports the fed document through the scanning means 96 to the document discharging means 97.

The scanning means 96, such as a CCD sensor module, scans the transported document. The document discharging means 97 discharges the transported document from the transporting path 90. Lastly, the document is transported out from the transporting path 90 and onto the document discharging tray 92.

Please refer to FIG. 2. A multiple documents feeding detection mechanism 100 is integrated with the document transporting means 95 of the scanner 9. The multiple documents feeding detection mechanism 100 has a transporting roller means 1 and a detecting means 2.

The transporting roller means 1 comprises an advancing roller 11, a retarding roller 13, a clutch 15, and a motor 17. The advancing roller 11 is against the retarding roller 13 for transporting one piece of document therebetween. The clutch 15 applies a resisting torque to the retarding roller 13. The motor 17 applies a driving torque to the advancing roller 11 for driving the advancing roller 11 to supply an urging torque to the retarding roller 13. The urging torque abuts against the resisting torque applied to the retarding roller 13 by the clutch 15.

Please refer to FIG. 2, if there is no document fed into a nip between the advancing roller 11 and the retarding roller 13, then the urging torque supplied from the advancing roller 11 will overcome the resisting torque applied to the retarding roller 13. Consequently, the retarding roller 13 frictionally connects to the advancing roller 11 and is urged into rotation by the urging torque.

At the nip, the advancing roller 11 exerts a forward tangential force Fa on the retarding roller 13 and the resisting torque T applied to the retarding roller 13. The forward tangential force Fa is shown as Fa=Ua−r*N, wherein Ua−r is the friction coefficient of the advancing roller 11-to-retarding roller 13, N is the normal force for urging the advancing roller 11 and the retarding roller 13 together.

While there is no document fed into the nip, then the urging torque supplied from the advancing roller 11 is shown as Fa*Rb. If the retarding roller 13 is frictionally urged into rotation with the advancing roller 11 by the urging torque Fa*Rb, then the urging torque Fa*Rb must overcome the resisting torque T applied to the retarding roller 13. That is, Fa*Rb>T, wherein Rb is a radius of the retarding roller 13.

Please refer to FIG. 3, while a single document 3 is fed into the nip, the urging torque supplied from the advancing roller 11 and transmitted through the document 3 will overcome the resisting torque applied to the retarding roller 13. Consequently, the retarding roller 13 frictionally connects to the document 3 and is urged into rotation with the advancing roller 11 by the urging torque.

If a single document 3 is fed into the nip, the advancing roller 11 exerts a forward tangential force Fb on an upper surface of the document 3, meanwhile, the retarding roller 13 exerts a reverse tangential force Fc on a lower surface of the document 3. The forward tangential force Fb is shown as Fb=Ua−p*N, wherein Ua−p is the friction coefficient of the advancing roller 11-to-the document 3. The reverse tangential force Fc is shown as Fc=Ub−p*N, wherein Ub−p is the friction coefficient of the retarding roller 13-to-the document 3.

While the forward tangential force Fb overcomes the reverse tangential force Fc, the document 3 and the retarding roller 13 will be together urged into rotation with the advancing roller 11. That is, Fb>Fc, or Ua−p*N>Ub−p*N. The friction coefficient Ua−p of the advancing roller 11-to-the document 3 is greater than the friction coefficient Ub−p of the retarding roller 13-to-the document 3.

Thus, the urging torque supplied from the advancing roller 11 and transmitted through the document 3 is shown as Fc*Rb. While the retarding roller 13 frictionally connects to the document 3 and is urged into rotation with the advancing roller 11 by the urging torque Fc*Rb, the urging torque Fc*Rb must overcome the resisting torque T applied to the retarding roller 13. That is, Fc*Rb>T.

Please refer to FIG. 4, if two or more documents 3, 3′ are fed into the nip, then the urging torque supplied from the advancing roller 11 and transmitted through the document 3 to document 3′ can not overcome the resisting torque applied to the retarding roller 13. Consequently, the retarding roller 13 frictionally connects to the document 3′ and is remained in standstill by the resisting torque applied to the retarding roller 13.

If multiple documents 3, 3′ are fed into the nip, the upper document 3 exerts a forward tangential force Fe on the lower document 3′, and the retarding roller 13 exerts a reverse tangential force Fd on the lower document 3′. The forward tangential force is shown as Fe=Up*N, wherein Up is the friction coefficient of the upper document 3-to-the lower document 3′. The reverse tangential force Fd is shown as Fd=Ub−p*N, wherein Ub−p is the friction coefficient of the retarding roller 13-to-the lower document 3′.

While the forward tangential force Fe can not overcome the reverse tangential force Fd, the lower document 3′ is remained at the nip. That is, Fd>Fe, or Ub−p*N>Up*N. The friction coefficient Ub−p of retarding roller 13-to-the lower document 3′ is greater than the friction coefficient Up of the upper document 3-to-the lower document 3′.

In order to remain the lower document 3′ at the nip and the retarding roller 13 in standstill by the resisting torque applied to the retarding roller 13, the urging torque supplied from the upper document 3 must be less than the resisting torque applied to the retarding roller 13. That is, Fe*Rb<T.

Therefore, if there is no document fed into the nip, then the retarding roller 13 frictionally connects to the advancing roller 11 and is urged into rotation with the advancing roller 11. If one piece of document 3 is fed into the nip, then the retarding roller 13 frictionally connects to the document 3 and is urged into rotation with the advancing roller 11. If multiple documents 3, 3′ are fed into the nip, then the retarding roller 13 is remained in standstill and the lower document 3′ is remained at the nip.

Please refer to FIG. 5, the first exemplary embodiment of the detecting means 2 is shown. The detecting means has a disk 20, a shaft 21, a light source 22, a sensor 23 and a micro processor unit (MPU) 24. The disk 20 having an obstructing portion and a penetrating portion is located on one lateral side of the retarding roller 13.

More specifically, the obstructing portion is the body of the disk 20 and the penetrating portion has a plurality of through holes 25 formed at the rim of the disk 20. The shaft 21 coaxially interconnects the retarding roller 13 and the disk 20. Two opposite sides of the disk 20 are located between the light source 22 and the sensor 23 respectively. The light source 22 and the sensor 23 are aligned with each other and aligned together with the through holes 25. The MPU 24 connects to the sensor 23.

Consequently, the disk 20 of the detecting means 2 is urged into rotation with the retarding roller 13 of the transporting means 1. If only one single document 3 is fed into the nip, then the disk 20 will be urged into rotation with the retarding roller 13. If multiple documents 3, 3′ are fed into the nip, then the disk 20 will be remained in standstill.

Please refer to FIG. 5 and FIG. 6. While the disk 20 is urged into rotation by the retarding roller 13, the light source 22 radiates a beam radiating through the through holes 25 of the disk 20 and then obstructed by the disk 20 repeatedly. The sensor 23 receives an input signal detected as staggered light and dark, and then sends out a continuous pulse signal (5 Voltage) to the MPU 24. The MPU 24 determines that a single document 3 being fed into the nip between the advancing roller 11 and the retarding roller 13 of the transporting means 1.

Please refer to FIG. 7 and FIG. 8. While the disk 20 is remained in standstill, the light source 22 radiates a beam radiating through the through holes 25 of the disk 20 or obstructed by the disk 20. Thus, the sensor 23 receives an input signal detected as all light or all dark, and then sends out a constant signal to the MPU 24.

More specifically, while the sensor 23 receives an input signal detected as all light, the sensor 23 sends out a high potential (5 Voltage) signal. While the sensor 23 receives an input signal detected as all dark, the sensor 23 sends out a low potential (0 Voltage) signal. The MPU 24 determines that the multiple documents 3, 3′ are fed into the nip according to the received low potential signal or the received high potential signal from the sensor 23.

Please refer to FIG. 9, the second exemplary embodiment of the optical detecting means 2 is shown. The disk 20 has staggered low reflecting portions 26 and high reflecting portions 27. The light source 22 and the sensor 23 are located at the same side of the disk 20. Specifically, the low reflecting portions 26 are made of black material and the high reflecting portions 27 are made of white material.

While the disk 20 is urged into rotation by the retarding roller 13, the light source 22 radiates a beam to the disk 20, which is reflected by the high reflecting portions 27 and then absorbed by the low reflecting portion 26 repeatedly. Thus, the sensor 23 receives an input signal detected as staggered light and dark, and then sends out a continuous pulse signal to the MPU 24.

While the disk 20 is remained in standstill with the retarding roller 13, the light source 22 radiates a beam to the disk 20 which is reflected by the high reflecting portions 27 or absorbed by the low reflecting portions 26. As described above, the MPU 24 determines whether the multiple documents 3, 3′ are fed into the nip according to the received signal from the sensor 23. Furthermore, the beam radiated from the light source 22 can be visible, such as while light, or invisible, such as infrared rays.

In order to urge the retarding roller 13 into rotation with the advancing roller 11 while a single document 3 fed into the nip, and remain the retarding roller 13 in standstill while multiple documents 3, 3′ is fed into the nip, it is assumed that Ua−p>Ub−p>Up. Moreover, in order to remain the lower document 3′ at the nip while multiple documents 3, 3′ is fed into the nip, it is assumed that Fd*Rb>T>Fe*Rb.

The multiple documents feeding mechanism 100 determines multiple documents feeding by monitoring the action of the transporting roller means 1. The retarding roller 13 is urged into rotation with the advancing roller 11 while a single document fed into the nip. In stead, the retarding roller 12 is remained in standstill while multiple documents 3, 3′ fed into the nip, by comparing the friction coefficient of the surface of the advancing roller 11 land the friction coefficient of the surface of the retarding roller, as well as the driving torque supplied by the motor 17 and the resisting torque supplied by the clutch 17.

The detecting means 2 detects multiple documents feeding via monitoring the action of the transporting roller means 1. The disk 20 is driven into rotation with the retarding roller 13. The disk 20 has a first light tuning portion, such as the obstructing portion and the low reflecting portion 26 described above, and a second light tuning portion, such as the penetrating portion and the high reflecting portion 27 described above.

The light source 22 radiates a beam to the disk 20, which is received by the sensor 23. The sensor 23 sends a first signal, such as continuous pulse signal described above, according to the received variable intensity of the beam alternately tuned by the first tuning portion and the second tuning portion. In this embodiment, the input signal is detected as staggered light and dark, due to movement of the disk.

The sensor 23 sends a second signal, such as the low potential signal or the high potential signal, according to the received constant intensity of the beam which is tuned by one of the first tuning portion and the second tuning portion. In this embodiment, the input signal detected as all dark and all light described above, due to standstill of the disk.

The MCU 24 determines the multiple documents 3, 3′ are fed into the nip according to the received first signal from the sensor 23. The MCU 24 determines that there is only one single document fed into the nip according to the received second signal from the sensor 23. Therefore, the multiple documents feeding detection mechanism 100 has the following advantages.

Firstly, the thickness of the document fed into the multiple documents feeding mechanism 100 is much more than ever. Secondly, the multiple documents feeding mechanism 100 can determine whether the multiple documents feeding being caused by monitoring the movement of the retarding roller 13.

It is capable of avoiding the influence by the dust. Thirdly, the multiple documents feeding detection mechanism 100 is integrated with the transportation means for being conveniently used. Fourthly, the cost of the detecting means 2 of the multiple documents feeding detection mechanism 100 is cheaper than the cost of the ultrasound beam generator and receiver.

Furthermore, the present invention is not limited to the embodiments described above; various additions, alterations and the like may be made within the scope of the present invention by a person skilled in the art. For example, respective embodiments may be appropriately combined. 

1. A multiple documents feeding detection mechanism, comprising: a driving means; an advancing roller driven by the driving means; a retarding roller being against the advancing roller for transporting one piece of document therebetween; a clutch providing a resisting torque to the retarding roller; and a detecting means monitoring an action of the retarding roller for determining multiple documents being fed into a nip between the advancing roller and the retarding roller; wherein the friction coefficient of a surface of the advancing roller is greater than the friction coefficient of a surface of the retarding roller, and the friction coefficient of the surface of the retarding roller is greater than the friction coefficient of the fed documents, wherein if the multiple documents are fed into the nip, then the friction of the surface of the retarding roller-to-the document will be greater than the friction between the documents to remain the retarding roller be in standstill.
 2. The multiple documents feeding detection mechanism as claimed in claim 1, wherein the detecting means comprises a disk, at least a through hole penetrating the disk, a shaft coaxially interconnecting the retarding roller and the disk, a light source located at one side of the disk and aligned with the through hole, a sensor located at the other side of the disk and aligned together with the light source and the through hole, and a processor connected to the sensor.
 3. The multiple documents feeding detection mechanism as claimed in claim 1, wherein the detecting means comprises a disk having staggered a low reflecting portion and a high reflecting portion on one surface thereof, a shaft coaxially interconnecting the retarding roller and the disk, a light source and a sensor located at the same side of the disk and facing the high reflecting portion and the low reflecting portion, and a processor connected to the sensor.
 4. The multiple documents feeding detection mechanism as claimed in claim 3, wherein the low reflecting portion is made of a black material, the high reflecting portion is made of a white material.
 5. A multiple documents feeding detection method, comprising: providing an advancing roller applied a driving torque; providing a retarding roller applied a resisting torque less than the driving torque, and against the advancing roller for transporting one piece of document; monitoring an action of the retarding roller by a detecting means to detect whether the multiple documents being fed into a nip between the advancing roller and the retarding roller; wherein the friction coefficient of a surface of the advancing roller is greater than the friction coefficient of a surface of the retarding roller, and the friction coefficient of the surface of the retarding roller is greater than the friction coefficient of the fed documents, wherein if multiple documents are fed into the nip, then the friction of the surface of the retarding roller-to-the document is greater than the friction between the documents to remain the retarding roller be in standstill.
 6. The method as claimed in claim 5, further comprises: providing a disk acted with the retarding roller, an obstructing portion and a penetrating portion formed on the disk; locating a light source at one side of the disk, locating a sensor at the other side of the disk, the light source and the sensor aligned with each other, and aligned together with the penetrating portion of the disk; connecting a processor to the sensor; wherein if the sensor receives a beam from the light source alternately radiating through the penetrating portion and then obstructed by the obstructing portion of the disk, then the processor determines that there is only one single document fed into the nip; if the sensor receives the beam from the light source constantly radiating through the penetrating portion of the disk or constantly obstructed by the obstructing portion of the disk, then the processor determines there are multiple documents fed into the nip.
 7. The multiple documents feeding detection method as claimed in claim 6, wherein said beam radiated from said light source is one of a visible beam and an invisible beam.
 8. The method as claimed in claim 6, wherein if the beam radiating through the penetrating portion and obstructed by the obstructing portion of the disk alternately, then the sensor sends out a continuous pulse to the processor for determining that there is one single document fed into the nip; if the beam obstructed by the obstructing portion constantly due to standstill of the disk, then the sensor sends out a first potential signal to the processor for determining there are multiple document fed into the nip; if the beam radiating through the penetrating portion constantly due to the standstill of the disk, then the sensor sends out a second potential signal to the processor for determining there are multiple documents fed into the nip.
 9. The method as claimed in claim 8, wherein the first potential signal is a low potential signal and the second potential signal is a high potential signal.
 10. The method as claimed in claim 5, further comprises: providing a disk acted with the retarding roller, a low reflecting portion and a high reflecting portion formed on the disk; locating a light source radiating a beam, and a sensor at one side of the disk; connecting a processor to the sensor; wherein while the sensor receives a variable intensity of the beam reflected by the high reflecting portion and then be absorbed by the low reflecting portion of the disk repeatedly due to movement of the disk, the processor determines that there is only one single document fed into the nip; wherein the processor determines multiple document are fed into the nip while the sensor receives a constant intensity of the beam reflected by one of the low reflecting portion and the high reflecting portion of the disk.
 11. The method as claimed in claim 10, wherein the sensor sends out a continuous pulse signal to the processor according to the received variable intensity of the beam, and then the processor determines that there is only one single document fed into the nip according to received continuous pulse signal; wherein the sensor sends out a constant signal to the processor according to the received constant intensity of the beam, and then the processor determines multiple document fed into the nip according to received constant signal.
 12. The method as claimed in claim 10, wherein the high reflecting portion is made of a white material, the low reflecting portion is made of a black material.
 13. A multiple documents feeding detection mechanism, comprising: an advancing roller having a surface with a first friction coefficient; a driving mean providing a driving torque to the advancing roller; a retarding roller having a surface with a second friction coefficient, against the advancing roller for transporting one piece of document with a third friction coefficient; a clutch providing a resisting torque to the retarding roller, the resisting torque being less than the driving torque, if there are multiple documents fed into the nip, then the retarding roller remains in standstill; a disk driven into action with the retarding roller, having a first tuning portion and a second tuning portion; a source radiating a beam to the disk; a sensor sending a first signal according to a received variable intensity of the beam radiated from the source and then alternately tuning by the first tuning portion and the second tuning portion due to movement of the disk, and sending a second signal according to a received constant intensity of the beam radiated from the source and then tuning by one of the first tuning portion and the second tuning portion due to standstill of the disk; and a processor connected to the sensor, determining multiple documents fed into the nip according to a received second signal from the sensor, and determining that there is one single document fed into the nip according to a received first signal from the sensor.
 14. A multiple documents feeding detection method, comprising: providing an advancing roller; applying a driving torque to the advancing roller; providing a retarding roller against the advancing roller for transporting one piece of document; applying a resisting torque to the retarding roller; setting the friction coefficient of a surface of the advancing roller be greater than the friction coefficient of a surface of the retarding roller, the driving torque be greater than the resisting torque, to urge the retarding roller into rotation when only one single document fed into a nip between the advancing roller and the retarding roller, and urge the retarding roller be in standstill when multiple documents fed into the nip; providing a disk with a first tuning portion and a second tuning portion, driven into action with the retarding roller; radiating a beam to the disk; determining there is one single document fed into the nip according to a received variable intensity of the beam alternately tuned by the first tuning portion and the second tuning portion due to the movement of the disk; determining that there are multiple documents fed into the nip according to a received constant intensity of the beam tuned by one of the first tuning portion and the second tuning portion due to standstill of the disk. 